Lanthanum cerium terbium phosphate (LAP) phosphors can be described by the general formula (La.sub.x, Ce.sub.y, Tb.sub.z) PO.sub.4 where x is from about 0.73 to about 0.37, y is from about 0.17 to about 0.45 and z is from about 0.10 to about 0.18. Examples of these phosphors can be found in U.S. Pat. Nos. 5,106,532 and 5,132,042 which are incorporated herein by reference. LAP phosphors are green-emitting when stimulated by ultra-violet (UV) radiation which makes them useful in the manufacture of certain fluorescent lamps especially energy efficient lamps (&gt;75 lumens/watt) and high CRI (&gt;70) lamps. Typically, the green-emitting LAP phosphors are blended with other materials such as alumina or other phosphors and coated onto the interior surface of fluorescent lamps using conventional coating techniques. As a general principle in fluorescent lamp manufacturing, the particle size of a phosphor affects the amount of coating needed to produce an optimal coating for a particular application. Phosphors having smaller particle sizes can provide equivalent coverage densities at lower powder weights than larger sized phosphors. Such lower powder weights reduce the manufacturing cost of the lamp.
However, a reduction in the size of a phosphor usually results in lower emission brightness because of the higher intrinsic reflection coefficient associated with the smaller particles. Additionally, because it is difficult to control the phosphor particle size during synthesis, the general practice of making a smaller sized phosphor involves milling the phosphor after synthesis. The milling process results in a further loss in brightness. Thus, the loss of brightness from the combination of milling and smaller particle size generally neutralizes the economic advantage of reducing the phosphor particle size.
Thus, it would be a significant advantage to produce a smaller size LAP phosphor without the concomitant loss in brightness.